In industry the use of metal products manufactured by compacting and sintering iron-based powder compositions is becoming increasingly widespread. The quality requirements of these metal products are continuously raised, and as a consequence new powder compositions having improved properties are developed. One of the most important properties of the final, sintered products is the density and dimensional tolerances, which above all have to be consistent. Problems with size variations in the final product often originates from inhomogenities in the powder mixture to be compacted. These problems are especially pronounced with powder mixtures including pulverulent components, which differ in size, density and shape, a reason why segregation occurs during the transport, storage and handling of the powder composition. This segregation implies that the composition will be non-uniformly composed, which in turn means that parts made of the powder composition are differently composed and consequently have different properties. A further problem is that fine particles, particularly those of lower density such as graphite, cause dusting in the handling of the powder mixture.
The small particle size of additives also create problems with the flow properties of the powder, i.e. the capacity of the powder to behave as a free-flowing powder. An impaired flow manifests itself in increased time for filling dies with powder, which means lower productivity and an increased risk of variations in density in the compacted component, which may lead to unacceptable deformations after sintering.
Attempts have been made at solving the problems described above by adding different binding agents and lubricants to the powder composition. The purpose of the binder is to bind firmly and effectively the small size particles of additives, such as alloying components, to the surface of the base metal particles and, consequently, reduce the problems of segregation and dusting. The purpose of the lubricant is to reduce the internal and external friction during compaction of the powder composition and also reduce the ejection force, i.e. the force required to eject the finally compacted product from the die.
Various organic binding agents are disclosed in for example the U.S. Pat. No. 4,483,905 (Engstrom) which teaches the use of a binding agent that is broadly described as being of “a sticky or fat character”. The U.S. Pat. No. 4,676,831 (Engstrom) discloses the use of certain tall oils as binding agents. Furthermore the U.S. Pat. No. 4,834,800 (Semel) discloses the use of certain film-forming polymeric resins that are insoluble or substantially insoluble in water as binding agents.
Other types of binding agents set forth in the patent literature are polyalkylene oxides having molecular weights of at least about 7000, which are disclosed in the U.S. Pat. No. 5,298,055 (Semel). Combinations of dibasic organic acid and one or more additional components such as solid polyethers, liquid polyethers, and acrylic resins as binding agents are disclosed in the U.S. Pat. No. 5,290,336. Binding agents that can be used with high temperature compaction lubricants are disclosed in the U.S. Pat. No. 5,368,630 (Luk).
Furthermore, the U.S. Pat. No. 5,480,469 (Storström) provides a brief review of the use of binding agents in the powder metallurgy industry. The patent notes that it is important to have not only a powder composition that has the alloying powder adhered to the iron-based powder by way of the binding agent, but to also have a lubricant present to achieve adequate compressibility of the powder composition within the die and to decrease the forces required to remove the part from the die.
Specifically, the U.S. Pat. No. 5,480,469 teaches a method for binding additives in an iron-based powder metallurgical mixture to the iron or iron-based powder particles by the use of a diamide wax binder. In order to achieve an effective binding between the iron or iron-based particles and the additive particles the powder metallurgical mixture including the binder is mixed and heated to about 90-160° C. during mixing and melting of the binder, and subsequently the mixture is cooled during mixing, until the binder has solidified. By this method the flow and apparent density is substantially improved and the problem with dusting can be reduced or eliminated.
A property of a powder mix which is not specifically discussed in the U.S. Pat. No. 5,480,469 is the lubricating property. This property is of particular importance when components having high density and/or a complex shape are required. In connection with the production of such components it is essential that the lubricating properties of the used powder metallurgical mixture are good which in turn means that the energy needed in order to eject to component from the die, i.e. the ejection energy, should be low which is a pre-requisite for a satisfactory surface finish of the ejected component, i.e. a surface finish without any scratches or other defects.
We have now developed a new iron or iron based composition which is distinguished by low segregation and low dusting, good flow and high apparent density and which is also distinguished by good lubricating properties i.e. properties which are all important for powders to be compacted and sintered to high quality products.